Born in Wisconsin, Harold Babcock finished high school in Los Angeles and earned his bachelor’s degree in electrical engineering at the University of California in 1906. In 1909 he became one of the first staff members of the Mt. Wilson Observatory, where he remained until retirement in 1948, after which he continued research and instrument-building. In the early years he made precision measurements of spectra in the laboratory and participated in solar research with George E. Hale. Babcock’s precise laboratory studies of atomic spectra allowed others to identify the first “forbidden” lines in the laboratory and to discover the rare isotopes of oxygen. With C.E. St. John he greatly improved the precision of the wavelengths of some 22,000 lines in the solar spectrum, referring them to newly-determined standards, and extending measurements into the ultraviolet and infrared. With his son, Horace W. Babcock, he measured the distribution of magnetic fields over the solar surface to unprecedented precision. He supervised the construction of a ruling engine and ruled excellent large gratings, including those used in the coudé spectrographs of the 100 and 200-inch telescopes.